1. Technical Field
The present invention relates to a method of diluting a polymer solution and more particularly to a method of diluting a concentrated polymer solution for use in a hydrocarbon recovery process.
2. Description of Related Art
Dilute aqueous polymer solutions are commonly used in secondary and tertiary hydrocarbon recovery processes. For example, the dilute polymer solutions are employed as displacement fluids in immiscible floods and mobility buffers in miscible floods of subterranean hydrocarbon-bearing formations.
The water-soluble polymers used in these processes are usually manufactured in solution at locations remote from the oil field injection site. Therefore, it is necessary to transfer the polymer from the point of manufacture to the point of use. The polymer is transported in one of two forms, either the solution form as manufactured or a dried solid form.
The polymer is preferably maintained in the solution form because the drying process can detrimentally degrade the polymer. However, it is generally necessary to dilute the polymer solution before injection into a formation because most polymer solutions are too concentrated as manufactured for subterranean applications. Dilution is preferably performed at the injection site because transportation of a bulk dilute polymer solution from a remote dilution site to an injection site is relatively expensive.
Concentrated polymer solutions are difficult to dilute because the concentrated solution can have a gel-like character, even if the polymer is uncrosslinked. That is to say, a concentrated uncrosslinked polymer solution can exhibit an extreme resistance to flow similar to a cross-linked polymer gel which prevents the solution from rapidly and spontaneously dispersing in a dilution water, although the polymer may be characterized as water-soluble.
Known means of diluting a concentrated uncrosslinked polymer solution with an aqueous diluent include agitation of the solution in the presence of the diluent using stirrers, blenders or the like to physically break up the gel-like solution and disperse it in the diluent. Agitation methods are usually rapid, but they can degrade the polymer to such a degree that the resulting dilute polymer solution is unacceptable for many hydrocarbon recovery applications.
Other means of dilution rely on passive diffusion of the diluent into the polymer solution. Tanaka describes principles of polymer gel dilution by diffusion, i.e., swelling, in "Gels", Scientific American, v. 244, No. 1, pp. 124-138 (January 1981). Diffusion methods are comparatively slow for oil field use, especially when the aqueous diluent has a high salt concentration, such as a formation brine. Despite their slowness, diffusion-type dilution processes are preferred because degradation of the polymer is minimized.
U.S. Pat. No. 4,402,916 to Luetzelschwab describes an apparatus utilizing a diffusion method to dilute a concentrated uncrosslinked polymer solution with an aqueous diluent. The apparatus promotes rapid diffusion of the diluent into the gel-like polymer solution by sequentially increasing the surface area of the solution. The apparatus is relatively effective for rapid dilution of the polymer solution with a fresh water diluent, but is less effective for rapid dilution when the diluent is a brine.
A method is needed to accelerate the dilution of a gel-like concentrated water-soluble polymer solution with an aqueous diluent and particularly with a brine diluent. A method is needed which accelerates the dilution rate of the polymer solution without substantially degrading the polymer so that the polymer retains favorable properties for hydrocarbon recovery applications.